Global demand for C3-C4 olefins herein also referred to as lower olefins is estimated to grow at 5-7% per year. Various processes have been studied to increase the yield to these lower olefins in fluid catalytic cracking (FCC) processes.
In FCC processes a preheated hydrocarbonaceous feedstock of a high boiling point range is brought into contact with a hot cracking catalyst in a riser. The feed is cracked into lower boiling products, such as dry gas, LPG, gasoline, and cycle oils. Furthermore, coke and non-volatile products deposit on the catalyst resulting in a spent catalyst. The riser exits into a separator wherein the spent catalyst is separated from the reaction products. In the next step the spent catalyst is stripped with steam to remove the non-volatile hydrocarbon products from the catalyst. The stripped catalyst is passed to a regenerator in which coke and remaining hydrocarbonaceous materials are combusted and wherein the catalyst is heated to a temperature required for the cracking reactions. Hereafter the hot regenerated catalyst is returned to the riser reactor zone. In operation, fresh catalyst must be added to the unit to make up for losses and to maintain the catalyst activity. Feeds that can be used for FCC processes are gas oils and residues and include straight run (atmospheric) gas oil, vacuum gas oil, and coker gas oils. Catalysts that can be used for FCC processes are generally based on zeolites, especially large pore synthetic faujasites, such as zeolites X and Y.
US-A-2002/0189973 describes a FCC process having increased production of light olefins, including propylene. To increase the production of light olefins at least two risers are being used feeding into a single separation-stripping vessel. The FCC feed is catalytically cracked to produce a crackate that comprises naphtha, propylene and other cracked products in a first riser, with recovery and recycle of at least a portion of the naphtha crackate as feed into the second riser, in which it is catalytically cracked into products comprising additional propylene. As cracking catalyst the same catalyst is used in both risers comprising large and medium pore, shape selective zeolite components.
The disadvantage of the process as disclosed in US-A-2002/0189973 is that it is only applicable for FCC units that have at least two riser reactors. Revamping of existing units with only one riser by building an extra riser is expensive. Moreover, it would be desirable to increase the yield to lower olefins further.
Hence, the object of the present invention is to provide a process with increased selectivity to C3 and C4 olefins.